Plants 'no buffer against global warming'

Natural ecosystems cannot be relied upon as a way of soaking up the carbon dioxide responsible for climate change, according to a study published in Science this week by a group of US researchers.

It has long been thought that, because plants need to absorb carbon dioxide in order to grow, they could be treated as a buffer against fossil fuel emissions containing the gas. But new research has turned this assumption on its head.

Researchers at Stanford University report that increased levels of carbon dioxide — when combined with the other effects of climate change — actually suppress growth rather than helping plants to flourish.

The three-year field experiment was unusual in looking at the effects of temperature, rainfall and nitrogen deposits, as well as carbon dioxide, in an attempt to mimic future climate conditions as accurately as possible. The researchers were surprised to find that while increases in carbon dioxide alone did indeed encourage plants to grow, when combined with these other factors it consistently dampened growth.

"To understand complex ecological systems, the traditional approach of isolating one factor and looking at that response, then extrapolating to the whole system, is often not correct," says Harold Mooney, co-author of the study. "On an ecosystem scale, many interacting factors may be involved."

The researchers are uncertain why this combination of factors has a suppressive effect. But one reason might be that excess carbon in the soil allows microbes to out-compete plants for nutrients.

The authors warn that the findings suggest that we shouldn't rely on increased plant growth to counteract emissions of carbon dioxide — the rationale for employing forest 'carbon sinks' to mitigate climate change.

"Our experiment shows that we can't count on the natural world, the unmanaged world, to save us by pulling down all the atmospheric carbon dioxide," says co-author Christopher B. Field.